Sand-filled electric fuses

ABSTRACT

In a sand-filled electric fuse, the fuse element and the surrounding sand consist of or contain such materials that mutually react chemically in an exothermic process at temperatures above the melting point of the fuse element. The fuse element may predominantly consist of aluminum or magnesium and the filler material may be quartz sand.

United States Patent Nielsen 51 May 9, 1972 [54] SAND-FILLED ELECTRIC FUSES [72] Inventor: Per Qstergaard, Nielsen, Copenhagen,

Denmark [73} Assignee: Aktieselskabet Laur. Knudsen, Nordisk Elektricitets Selskab, Copenhagen, Denmark [22] Filed: July 9, 1970 [21] Appl. No.: 53,348

[30] Foreign Application Priority Data May 4, 1970 Denmark ..2250/70 [52] U.S. Cl ..337/290 [51] Int. Cl. ..H01h 85/38 [58] Field ofSearch ..337/l58,273, 276, 290, 296

[56] References Cited UNITED STATES PATENTS 1,856,701 5/1932 Gerdien ..337/29O Primary ExaminerGeorge Harris Assistant E.\'aminerF. E. Bell Attorney-Watson, Cole, Grindle & Watson [57] ABSTRACT In a sand-filled electric fuse, the fuse element and the surrounding sand consist of or contain such materials that mutually react chemically in an exothermic process at temperatures above the melting point of the fuse element. The fuse element may predominantly consist of aluminum or magnesium and the filler material may be quartz sand.

3 Claims, 2 Drawing Figures PATENTEUMAY 9 I972 3, 662 31 0 Fig- 1 ATTORNEY SAND-FILLED ELECTRIC FUSES BACKGROUND OF THE INVENTION This invention relates to a sand-filled electric fuse, that is a fuse of the kind where the breaking of the current takes place by means of an electric arc burning in a closely packed granular insulating material of uniform or varying grain size. Within the art, such material is usually referred to as sand irrespective of its exact chemical composition.

The operation of electric fuses can be divided into two periods, the melting period or pre-arcing period which runs from the moment when the current, e.g. a short-circuit current, begins to flow, up to the moment when the fuse element melts and opens the metallic circuit, and the arcing period which runs from then on until the current is ultimately broken.

When the fuse element melts and the arc is established it is important for the achievement of a satisfactory operation of the fuse that the voltage established across the arc has and retains a suitable magnitude in relation to the voltage of the circuit in which the fuse is inserted.

In order to ensure a suitable arc voltage magnitude two different approaches may be distinguished. The fuse can be designed in such a manner that a multitude of serially connected arcs are formed, or in such a manner that a long arc is formed, but these two approaches are not mutually exclusive. There are other conditions too, that contribute to the increase of the arc voltage, such as the cooling from the filler material surrounding the arc and the increase of pressure in the limited space in which the fuse element and the filler material is enclosed.

A long arc can be achieved by causing a long cylindrical wire to melt, but this method has certain disadvantages, e.g. that it leads to a rather slowly melting fuse. It is known to avoid these disadvantages by the use of a fuse element having a reduced cross section at one or more locations. In the case of an overcurrent, such a reduced cross section will melt first, and thereafter the arc will be elongated by the progressive melting of more and more of the fuse element.

SUMMARY OF THE INVENTION It is the aim of this invention to devise methods for promoting the elongation of the arc during the arcing period, thereby to increase the arc-voltage and thus to shorten the arcing period of an electric fuse of the kind mentioned.

According to this invention this is achieved by surrounding the fuse element by a filling consisting of materials that will cause a chemical reaction of an exothermic character at temperatures above the melting temperature of the fuse element.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows a fuse element of known design, and FIG. 2 shows a diagrammatic presentation of the voltage across the arc.

DESCRIPTION OF THE PREFERRED EMBODIMENT The fuse element, of which a part is shown in figure 1, consists of a strip or ribbon of metal 11 which at a suitable location is pierced by a hole 12, by which the cross section is reduced. Such a reduction constitutes a weakening of the fuse element at the location considered with the effect that the fuse element is more prone to melt at such a location. A weakening of the fuse element may as well be obtained by any other known method, e.g. byv locally increasing the electric resistance by alloying, by a reduced cooling, or by purely random variations of the environment. On the occurrence of an overcurrent the fuse element will melt first at a weakened location, whereby a short are will be formed.

However, the short are formed represents a certain voltage, and with the current flowing it represents a release of energy. The arc voltage can be considered to be composed of three parts as illustrated in FIG. 2.

In the immediate proximity of the end points of the are I there are two practically constant voltage drops, the so-called cathode drop 1 and anode drop 2, and in the arc in between, in the plasma, there is a plasma drop 3, which is approximately proportional to the length of the arc.

The characteristic property of the two voltage drops 1 and 2, the cathode drop and the anode drop, is the fact that they appear in the immediate neighborhood of the surface of the fuse element. Thus the energy associated with these voltage drops is released at and absorbed by the fuse element itself, in the form of heat conducted to the portion of the fuse element immediately behind the surface. The result is that this portion of the fuse element is heated to melting. Since cathode drop and anode drop are essentially constant values, the energy released will be proportional to the current multiplied by the time elapsed, or in other words proportional to the integral of the current over the interval of time considered.

If the heating is taken to be adiabatic, and the heat contributed from the metallic resistance of the fuse element is neglected, the elongation of the arc will be proportional to the integral of the current over the time elapsed. As this is in fact a law of nature it is not possible by conventional means to promote the elongation, and it is found that any variation introduced with the object of increasing the speed, by which the arc is elongated, is accompanied by other variations with the opposite tendency.

In the case where only a single arc is involved it is known to cause a further elongation of the are by mechanical means, viz. by pulling the two parts of the fuse element apart. This is a practical solution for fuse elements surrounded by air, but not so practical when the fuse element is surrounded by a closely packed granular material.

However, by introducing an exothermic chemical process of the metal of the fuse element and the surrounding filler, which process takes place at temperatures above the melting point of the fuse element, as proposed according to the invention, there will be an extra contribution of energy available for the melting of the fuse element, and consequently the elongation of the arc will proceed faster. After the melting of the weakened part of the fuse element the electric and the chemical energy will cause a progressive destruction of the remaining part of the fuse element, and the electrically conducting metal of the fuse element will be transformed into an electrically insulating chemical compound.

It is a condition for the success of the invention that the chemical reaction does not take place at any of the values of temperature that may prevail in the fuse during normal service, but only at temperatures existing under the influence of the arc at the end points of the arc, i.e. temperatures above the melting point of the base metal of the fuse element.

It is not a necessary condition for the invention that the fuse element has one or more weakened locations, but such have the practical advantage of providing well defined starting points for the arc. For this purpose the weakening need only be slight, e.g. corresponding to a reduction of cross section of 10-20 percent. Preferably, the weakening should not exceed percent.

The invention is further explained by the following examples.

In one embodiment of the invention a fuse element of the type shown in FIG. 1 may be used. The fuse element consists essentially of aluminum and it is packed in quartz sand, which is an oxide of silicon. After the formation of an arc at the weakened spot on the fuse element, the chemically very active aluminum will react with the quartz sand. Practical experiments have shown cases in which the energy available for the destruction of the fuse element is more than doubled, i.e. the same elongation of the arc can be achieved in less than half the time.

Another embodiment of the invention utilizes a fuse element consisting essentially of magnesium, which in the same manner is caused to react with a filler of quartz sand.

exothermic reaction at a temperature greater than the melting point of said fusible element only with the striking of an arc in the region of said at least one weakened area. whereby the heat from said exothermic reaction elongates said are to extinguish it.

2. An electric fuse as in claim 1, in which the fuse element predominantly consists of aluminum.

3. An electric fuse as in claim 1, in which the fuse element predominantly consists of magnesium.

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2. An electric fuse as in claim 1, in which the fuse element predominantly consists of aluminum.
 3. An electric fuse as in claim 1, in which the fuse element predominantly consists of magnesium. 